Tag: M-Class Stars

Ultraviolet light is what you might call a controversial type of radiation. On the one hand, overexposure can lead to sunburn, an increased risk of skin cancer, and damage to a person’s eyesight and immune system. On the other hand, it also has some tremendous health benefits, which includes promoting stress relief and stimulating the body’s natural production of vitamin D, seratonin, and melanin.

And according to a new study from a team from Harvard University and the Harvard-Smithsonian Center for Astrophysics (CfA), ultraviolet radiation may even have played a critical role in the emergence of life here on Earth. As such, determining how much UV radiation is produced by other types of stars could be one of the keys to finding evidence of life any planets that orbit them.

Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO

Recent studies have indicated that UV radiation may be necessary for the formation of ribonucleic acid (RNA), which is necessary for all forms of life as we know it. And given the rate at which rocky planets have been discovered around red dwarf stars of late (exampled include Proxima b, LHS 1140b, and the seven planets of the TRAPPIST-1 system), how much UV radiation red dwarfs give off could be central to determining exoplanet habitability.

“It would be like having a pile of wood and kindling and wanting to light a fire, but not having a match. Our research shows that the right amount of UV light might be one of the matches that gets life as we know it to ignite.”

For the sake of their study, the team created radiative transfer models of red dwarf stars. They then sought to determine if the UV environment on prebiotic Earth-analog planets which orbited them would be sufficient to stimulate the photoprocesses that would lead to the formation of RNA. From this, they calculated that planets orbiting M-dwarf stars would have access to 100–1000 times less bioactive UV radiation than a young Earth.

As a result, the chemistry that depends on UV light to turn chemical elements and prebiotic conditions into biological organisms would likely shut down. Alternately, the team estimated that even if this chemistry was able to proceed under a diminished level of UV radiation, it would operate at a much slower rate than it did on Earth billions of years ago.

As Robin Wordsworth – an assistant professor at the Harvard School of Engineering and Applied Science and a co-author on the study – explained, this is not necessarily bad news as far as questions of habitability go. “It may be a matter of finding the sweet spot,” he said. “There needs to be enough ultraviolet light to trigger the formation of life, but not so much that it erodes and removes the planet’s atmosphere.”

Previous studies have shown that even calm red dwarfs experience dramatic flares that periodically bombard their planets with bursts UV energy. While this was considered to be something hazardous, which could strip orbiting planets of their atmospheres and irradiate life, it is possible that such flares could compensate for the lower levels of UV being steadily produced by the star.

This news also comes on the heels of a study that indicated how the outer planets of the TRAPPIST-1 system (including the three located within its habitable zone) might still have plenty of water of their surfaces. Here too, the key was UV radiation, where the team responsible for the study monitored the TRAPPIST-1 planets for signs of hydrogen loss from their atmospheres (a sign of photodissociation).

Artist’s impression of a sunset seen from the surface of an Earth-like exoplanet. Credit: ESO/L. Calçada

Compared to higher-mass stars that have shorter life spans, red dwarf stars are likely to remain in their main sequence for as long as six to twelve trillion years. Hence, red dwarf stars would certainly be around long enough to accommodate even a vastly decelerated rate of organic evolution. In this respect, this latest study might even be considered a possible resolution for the Fermi Paradox – Where are all the aliens? They’re still evolving!

But as Dimitar Sasselov – the Phillips Professor of Astronomy at Harvard, the Director of the Origins of Life Initiative and a co-author on the paper – indicated, there are still many unanswered questions:

“We still have a lot of work to do in the laboratory and elsewhere to determine how factors, including UV, play into the question of life. Also, we need to determine whether life can form at much lower UV levels than we experience here on Earth.”

As always, scientists are forced to work with a limited frame of reference when it comes to assessing the habitability of other planets. To our knowledge, life exists on only on planet (i.e. Earth), which naturally influences our understanding of where and under what conditions life can thrive. And despite ongoing research, the question of how life emerged on Earth is still something of a mystery.

If life should be found on a planet orbiting a red dwarf, or in extreme environments we thought were uninhabitable, it would suggest that life can emerge and evolve in conditions that are very different from those of Earth. In the coming years, next-generation missions like the James Webb Space Telescope are the Giant Magellan Telescope are expected to reveal more about distant stars and their systems of planets.

The payoff of this research is likely to include new insights into where life can emerge and the conditions under which it can thrive.

Studies of low-mass, ultra-cool and ultra-dim red dwarf stars have turned up a wealth of extra-solar planets lately. These include the discoveries of a rocky planet orbiting the closest star to the Solar System (Proxima b) and a seven-planet system just 40 light years away (TRAPPIST-1). In the past few years, astronomers have also detected candidates orbiting the stars Gliese 581, Innes Star, Kepler 42, Gliese 832, Gliese 667, Gliese 3293, and others.

The majority of these planets have been terrestrial (i.e. rocky) in nature, and many were found to orbit within their star’s habitable zone (aka. “goldilocks zone”). However the question whether or not these planets are tidally-locked, where one face is constantly facing towards their star has been an ongoing one. And according to a new study from the University of Washington, tidally-locked planets may be more common than previously thought.

The study – which is available online under the title “Tidal Locking of Habitable Exoplanets” – was led by Rory Barnes, an assistant professor of astronomy and astrobiology at the University of Washington. Also a theorist with the Virtual Planetary Laboratory, his research is focused on the formation and evolution of planets that orbit in and around the “habitable zones” of low-mass stars.

Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth (left side). Credit: Wikipedia

For modern astronomers, tidal-locking is a well-understood phenomena. It occurs as a result of their being no net transfer of angular momentum between an astronomical body and the body it orbits. In other words, the orbiting body’s orbital period matches its rotational period, ensuring that the same side of this body is always facing towards the planet or star it orbits.

Consider Earth’s only satellite – the Moon. In addition to taking 27.32 days to orbit Earth, the Moon also takes 27.32 days to rotate once on its axis. This is why the Moon always presents the same “face” towards Earth, while the side that faces away is known as the “dark side”. Astronomers believe this became the case after a Mars-sized object (Theia) collided with Earth some 4.5 billion years ago.

Aside from throwing up debris that would eventually form the Moon, the impact is believed to have struck Earth at such an angle that it gave our planet an initial rotation period of 12 hours. In the past, researchers have used this 12-hour estimation of Earth’s rotation as a model for exoplanet behavior. However, prior to Barnes’ study, no systematic examinations had ever been conducted.

Looking to address this, Barnes chose to address the long-held assumption that only smaller, dimmer stars could host orbiting planets that were tidally locked. He also considered other possibilities, which included slower or faster initial rotation periods as well as variations in planet size and the eccentricity of their orbits. What he found was that previous studies had been rather limited and only made allowances for one outcome.

Tidally-locked, rocky planets are common around low-mass, M-type (red dwarf) stars, due to their close orbits. Credit: M. Weiss/CfA

“Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks. And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no Moon and with an initial ‘day’ that was four days long, one model predicts Earth would be tidally locked to the sun by now.”

From this, he found that potentially-habitable planets that orbit very late M-type (red dwarf) stars are likely to attain highly-circular orbits about 1 billion years after their formation. Furthermore, he found that for the majority, their orbits would be synchronized with their rotation – aka. they would be tidally-locked with their star. These findings could have significant implications for the study of exoplanets formation and evolution, not to mention habitability.

In the past, tidally-locked planets were thought to have extremes climates, thus eliminating any possibility of life. As an example, the planet Mercury experiences a 3:2 spin-orbit resonance, meaning it rotates three times on its axis for every two orbits it completes of the Sun. Because of this, a single day on Mercury lasts as long as 176 Earth days, and temperature range from 100 (-173 °C; -279 °F) to 700 K (427 °C; 800 °F) between the day side and the night side.

For a tidally-locked planets that orbit close to their stars, it was believed this situation would be even worse. However, astronomers have since come to speculate that the presence of an atmosphere around these planets could redistribute temperature across their surfaces. Unlike Mercury, which has no atmosphere and experiences no wind, these planets could maintain temperatures that would be supportive to life.

In any case, this study is one of many that is putting constraints on recent exoplanet discoveries. This is especially important given that the detection and study of extra-solar planets is still in its infancy, and limited to largely indirect methods. In other words, astronomers make estimates of a planet’s size, composition and whether or not it has an atmosphere based on transits and the influence these planets have on their stars.

In the coming years, next-generations missions like the James Web Space Telescope and the Transiting Exoplanet Survey Satellites (TESS) are expected to improve this situation drastically. In addition to conducting more detailed observations on existing discoveries, they are also expected to uncover a wealth of more planets. If Barnes’ study is correct, the majority of those found will be tidally-locked, but that need not mean they are uninhabitable.

In February of 2017, a team of European astronomers announced the discovery of a seven-planet system orbiting the nearby star TRAPPIST-1. Aside from the fact that all seven planets were rocky, there was the added bonus of three of them orbiting within TRAPPIST-1’s habitable zone. As such, multiple studies have been conducted that have sought to determine whether or not any planets in the system could be habitable.

When it comes to habitability studies, one of the key factors to consider is the age of the star system. Basically, young stars have a tendency to flare up and release harmful bursts of radiation while planets that orbit older stars have been subject to radiation for longer periods of time. Thanks to a new study by a pair of astronomers, it is now known that the TRAPPIST-1 system is twice as old as the Solar System.

The study, which will be published in The Astrophysical Journal under the title “On The Age Of The TRAPPIST-1 System“, was led by Adam Burgasser, an astronomer at the University of California San Diego (UCSD). He was joined by Eric Mamajek, the deputy program scientist for NASA’s Exoplanet Exploration Program (EEP) at the Jet Propulsion Laboratory.

Together, they consulted data on TRAPPIST-1s kinematics (i.e. the speed at which it orbits the center of the galaxy), its age, magnetic activity, density, absorption lines, surface gravity, metallicity, and the rate at which it experiences stellar flares. From all this, they determined that TRAPPIST-1 is quite old, somewhere between 5.4 and 9.8 billion years of age. This is up to twice as old as our own Solar System, which formed some 4.5 billion years ago.

These results contradict previously-held estimates, which were that the TRAPPIST-1 system was about 500 millions yeas old. This was based on the fact that it would have taken this long for a low-mass star like TRAPPIST-1 (which has roughly 8% the mass of our Sun) to contract to its minimum size. But with an upper age limit that is just under 10 billion years, this star system could be almost as old as the Universe itself!

“Our results really help constrain the evolution of the TRAPPIST-1 system, because the system has to have persisted for billions of years. This means the planets had to evolve together, otherwise the system would have fallen apart long ago.”

The implications of this could be very significant as far as habitability studies are concerned. For one, older stars experience less in the way of flareups compared to younger ones. From their study, Burgasser and Mamajek confirmed that TRAPPIST-1 is relatively quiet compared to other ultra-cool dwarf stars. However, since the planets around TRAPPIST-1 orbit so close to their star, they have been exposed to billions of years of radiation at this point.

An artist’s depiction of planets transiting a red dwarf star in the TRAPPIST-1 System. Credit: NASA/ESA/STScl

As such, it is possible that most of the planets which orbit TRAPPIST-1 – expect for the outermost two, g and h – would probably have had their atmospheres stripped away – similar to what happened to Mars billions of years ago when it lost its protective magnetic field. This is certainly consistent with many recent studies, which concluded that TRAPPIST-1’s solar activity would not be conducive to life on any of its planets.

Whereas some of these studies addressed TRAPPIST-1s level of stellar flare, others examined the role magnetic fields would play. In the end, they concluded that TRAPPIST-1 was too variable, and that its own magnetic field would likely be connected to the fields of its planets, allowing particles from the star to flow directly onto the planets atmospheres (thus allowing them to be more easily stripped away).

However, the results were not entirely bad news. Since the TRAPPIST-1 planets have estimated densities that are lower than that of Earth, it is possible that they have large amounts of volatile elements (i.e. water, carbon dioxide, ammonia, methane, etc). These could have led to the formation of thick atmospheres that protected the surfaces from a lot of harmful radiation and redistributed heat across the tidally-locked planets.

Then again, a thick atmosphere could also have an effect akin to Venus, creating a runaway greenhouse effect that would have resulted in incredibly thick atmospheres and extremely hot surfaces. Under the circumstances, then, any life that emerged on these planets would have had to be extremely hardy in order to survive for billions of years.

Artist’s impression of the view from the most distant exoplanet discovered around the red dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser.

Another positive thing to consider is TRAPPIST-1’s constant brightness and temperature, which are also typical of M-class (red dwarf) stars. Stars like our Sun have an estimated lifespan of 10 billion years (which it is almost halfway through) and grow steadily brighter and hotter with time. Red dwarfs, on the other hand, are believed to exist for as much as 10 trillion years – far longer than the Universe has existed – and do not change much in intensity.

Given the amount of time it took for complex life to have emerged on Earth (over 4.5 billion years), this longevity and consistency could make red dwarf star systems the best long-term bet for habitability. Such was the conclusion of one recent study, which was conducted by Prof. Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA). And as Mamajek explained:

“Stars much more massive than the Sun consume their fuel quickly, brightening over millions of years and exploding as supernovae. But TRAPPIST-1 is like a slow-burning candle that will shine for about 900 times longer than the current age of the universe.”

NASA has also expressed excitement over these findings. “These new results provide useful context for future observations of the TRAPPIST-1 planets, which could give us great insight into how planetary atmospheres form and evolve, and persist or not,” said Tiffany Kataria, an exoplanet scientist at JPL. At the moment, habitability studies of TRAPPIST-1 and other nearby star systems are confined to indirect methods.

However, in the near future, next-generation missions like the James Webb Space Telescope are expected to reveal additional information – such as whether or not these planets have atmospheres and what their compositions are. Future observations with the Hubble Space Telescope and the Spitzer Space Telescope are also expected to improve our understanding of these planets and possible conditions on their surface.

It is good time to be an exoplanet hunter… or just an exoplanet enthusiast for that matter! Every few weeks, it seems, new discoveries are being announced which present more exciting opportunities for scientific research. But even more exciting is the fact that every new find increases the likelihood of locating a potentially habitable planet (and hence, life) outside of our Solar System.

And with the discovery of LHS 1140b – a super-Earth located approximately 39 light years from Earth – exoplanet hunters think they have found the most likely candidate for habitability to date. Not only does this terrestrial (i.e. rocky) planet orbit within its sun’s habitable zone, but examinations of the planet (using the transit method) have revealed that it appears to have a viable atmosphere.

Credit for the discovery goes to a team of international scientists who used the MEarth-South telescope array – a robotic observatory located on Cerro Tololo in Chile – to spot the planet. This project monitors the brightness of thousands of red dwarf stars with the goal of detecting transiting planets. After consulting data obtained by the array, the team noted characteristic dips in the star’s brightness that indicated that a planet was passing in front of it.

The MEarth-South telescope array, located on Cerro Tololo in Chile, searches for planets by monitoring the brightness of nearby, small stars. Credit: Jonathan Irwin

These findings were then followed up using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the ESO’s La Silla Observatory, located on the outskirts of Chile’s Atacama Desert. According to the their study – which appeared in the April 20th, 2017, issue of the journal Nature – the team was able to make estimates of the planet’s age, size, mass, distance from its star, and orbital period.

They estimate that the planet is at least five billion years old – about 500 million years older than Earth. It is also slightly larger than Earth – 1.4 times Earth’s diameter – and is considerably more massive, weighing in at a hefty 6.6 Earth masses. Since they were able to view the planet almost edge-on, the team was also able to determine that it orbits its sun at a distance of about 0.1 AU (one-tenth the distance between Earth and the Sun) with a period of 25 days.

However, since its star is a red dwarf, this proximity places it in the middle of the system’s habitable zone. But what was most exciting was the fact that the team was able to look for evidence of an atmosphere since the planet was passing in front of its star – something that has not been possible with many exoplanets. Because of this, they were able to conduct transmission spectroscopy measurements that revealed the presence of an atmosphere.

As Jason Dittmann – of the Harvard-Smithsonian Center for Astrophysics (CfA) and the lead author of the study – said in a CfA press release:

“This is the most exciting exoplanet I’ve seen in the past decade. We could hardly hope for a better target to perform one of the biggest quests in science — searching for evidence of life beyond Earth.”

Granted, this exoplanet is not as close as Proxima b, which orbits Proxima Centauri – just 4.243 light years away. And it certainly isn’t as robust a find as the TRAPPIST-1 system, with its seven rocky planets, three of which are located within its habitable zone. But compared to these candidates, the researchers were able to place solid constraints on the planet’s mass and density, not to mention the fact that they were able to observe an atmosphere.

The discovery of an exoplanet that orbits a red dwarf star and has an atmosphere is also encouraging in a wider context. Low-mass red dwarf stars are the most common star in the galaxy, accounting for 75% of stars in our cosmic neighborhood alone. They are also long-lived (up to 10 trillion years), and recent research indicates that they are capable of hosting large numbers of planets.

But given their variability and unstable nature, astronomers have expressed doubts as to whether or not planet orbiting them could retain their atmospheres for very long. Knowing that a terrestrial planet that orbits a red dwarf, is five billion years old, and still has an atmosphere is therefore a very good sign. But of course, simply knowing there is an atmosphere doesn’t mean that it is conducive to life as we know it.

“Right now we’re just making educated guesses about the content of this planet’s atmosphere,” said Dittman. “Future observations might enable us to detect the atmosphere of a potentially habitable planet for the first time. We plan to search for water, and ultimately molecular oxygen.”

This chart shows the location of the faint red star LHS 1140 in the faint constellation of Cetus (The Sea Monster). This star is orbited by a super-Earth exoplanet called LHS 1140b, which may be best place to look for signs of life beyond the Solar System. The star is too faint to be seen in a small telescope.

Hence, additional studies will be needed before this planet can claim the title of “best place to look for signs of life beyond the Solar System”. To that end, future space-based missions like the James Webb Space Telescope (which will launch in 2018), and ground-based instruments like the Giant Magellan Telescope and the ESO’s Extremely Large Telescope, will be especially well-suited!

In the meantime, the NASA/ESA Hubble Space Telescope will be conducting observations of the star system in the near future. These observations, it is hoped, will indicate exactly how much high-energy radiation LHS 1140b receives from its sun. This too will go a long way towards determining just how habitable the Super-Earth is.

And be sure to enjoy this video of the LHS 1140 star system, courtesy of the European Southern Observatory and spaceengine.org:

It’s referred to as the “Goldilock’s Zone”, but this area in space isn’t meant for sleepy or hungry bears – it’s the relative area in which life can evolve and sustain. This habitable region has some fairly strict parameters, such as certain star types and rigid distance limits, but new research shows it could be considerably larger than estimated.

In a study done by Manoj Joshi and Robert Haberle, the team considered the relationship which occurs between the radiation for red dwarf stars and a possible planet’s reflective qualities. Known as albedo, this ability to “bounce back” light waves has a great deal to do with surface layers, such as ice and snow. Unlike our G-type Sun, the M-class red dwarf is much cooler and produces energy at longer wavelengths. This means a great deal of the radiation is absorbed – rather than reflected – turning the ice and snow into possible liquid water. And, as we know, water is considered to be a primary requirement for life.

“We knew that red dwarfs emit energy at a different wavelength, and we wanted to find out exactly what that might mean for the albedo of planets orbiting these stars.” explained Dr. Joshi from the National Centre for Atmospheric Science, who carried out the research in collaboration with Robert Haberle from the NASA Ames Research Centre.

What makes this theory even more charming is that M-class stars make up a very substantial portion of our galaxy’s total population – meaning there’s even more possible Goldilock’s Zones yet to be discovered. Considering the lifespan of a red dwarf star also increases the chances – as well as the distance a planet would need to be located for these properties to happen.

“M-stars comprise 80% of main-sequence stars, and so their planetary systems provide the best chance for finding habitable planets, i.e.: those with surface liquid water. We have modelled the broadband albedo or reflectivity of water ice and snow for simulated planetary surfaces orbiting two observed red dwarf stars (or M-stars) using spectrally resolved data of the Earth’s cryosphere.” explains Joshi. “In addition, planets with significant ice and snow cover will have significantly higher surface temperatures for a given stellar flux if the spectral variation of cryospheric albedo is considered, which in turn implies that the outer edge of the habitable zone around M-stars may be 10-30% further away from the parent star than previously thought.”

Have we discovered planets around red dwarf stars? The answer is yes. In order to calculate the effects of radiation and albedo, the team chose to use similar M-class stars, Gliese 436 and GJ 1214, and applied it to a simulated planet with an average surface temperature of 200 K. Why that particular temperature? In this circumstance, it’s the temperature at which one bar of carbon-dioxide condenses – a rough indicator of the outer edge of a habitable zone. It is theorized that anything registering below this temperature is too cold to harbor life.

What the team found was high albedo planets register a higher surface temperature when exposed to longer wavelength radiation. This means ice and snow covered planets could exist much further away from a red dwarf parent star – as much as one third more the distance.

“Previous studies haven’t included such detailed calculations of the different albedo effects of ice and snow.” explains Joshi. “But we were a little surprised how big the effect was.”